THE CAMBRIAN DIAGENETIC REVOLUTION

In a preliminary qualitative review, the style of dolomitization, silicification, and phosphatization, as well as the fractional organic carbon burial, appear to change across the Precambrian-Cambrian transition. In general, primary carbonate fabrics are more commonly retained during Precambrian dolomitization and obliterated during Phanerozoic dolomitization; silicification shows similar trends. Precambrian silicification and phosphatization commonly occurred early in the paragenetic sequence and demonstrate a higher potential for the preservation of microfossils versus Phanerozoic examples. Carbon cycling models, based on well documented carbon isotopic records, indicate a drastic reduction in the fractional organic carbon burial across the Precambrian Cambrian boundary, as well (e.g., Hayes et al., 1999). These observations might suggest that a fundamental change in the early diagenetic environment occurred through the Precambrian-Cambrian transition. Of course, these are generalizations, as individual exceptions are well known.

The advent of infaunal activity by burrowing metazoa (in particular, active vertical burrowing) at the Precambrian-Cambrian transition heralds the change from Proterozoic-style sharp sediment water interface (firmground) to a Phanerozoic-style diffuse sediment water interface (mixedground), and may provide one potential mechanism of change in the early diagenetic environment. A predictable consequence of this substrate change would be deeper, enhanced oxygenation of the sediment with increased mixing via infaunal activity. The reduction in the fractional burial of organic carbon is observed and is consistent with deeper oxygenation of the sediment. It is possible that deeper oxygenation also affected the other diagenetic processes outlined above (the sulfur cycle, e.g., Shields et al., 1999, may have been affected, as well). Thus, the advent of a key evolutionary innovation (vertical burrowing) may be the ultimate cause of the change in early diagenetic environments across the Precambrian-Cambrian transition.